Thanks for all the replies and advice everyone!
I knew the boat had some problems when I got it- hence the low price I paid. I was told that there was a significant leak and I had initially presumed that this was related to a crazed area near the waterline. Fortunately this turned out to be no deeper than the gelcoat. However the hull was in poor condition around one of the self-bailer openings- the inner and outer mouldings had parted and the bolt holes for the bailers were oversized so that the heads would pull through into the layup. Not nice.
I squeezed epoxy into the crack between the hull mouldings, and then fitted the bailers with longer bolts and washers, to clamp the whole thing back together. This has the downside that there are bolts heads protuding from the hull. But, hey, I’m not racing!
Obviously this repair has not be entirely successful. However I did keep the boat afloat for eight weeks before I noticed it was a little low in the water and decided to pump out the buoyancy compartment. So the water ingress it not happening at a catastrophic rate by any means.
A second problem is that the self-bailers themselves (plastic super chute 90s) let a little water through, into the drain sumps under the thwart. I don’t really see any harm in this as the water simply reaches a stable level after a while.
What’s the best way of finding the leaks? The old fairy-liquid and pumped air trick? Or just support the boat really well and put some water inside?